The intervertebral disc (IVD) is a cartilaginous structure that resembles articular cartilage in its biochemistry, but shows degenerative and ageing changes earlier than does many other connective tissue in the body. This is clinically important because IVD degeneration has been implicated in the etiology of lower back pain in millions of people. Lower back pain is an important socioeconomic disease and one of the most expensive health care issues today. The total cost of the lower back disabilities is in the range of $50 billion per year in the United States. IVD degeneration is caused by the increase of stiffness in nucleus pulposus (NP), bulging of annulus fibrosus (AF) and wearing of end plate (EP) caused by the mechanical overstress to these IVD components, which are shown as a schematic in FIG. 1. Improvements in IVD replacement strategies and materials are a promising, though as yet unrealized therapy for IVD degeneration. NP replacement by viscoelastic gel is emerging as a possible minimally invasive approach for the treatment of degenerative IVD disease. Swelled (hydrogel based material) and unswelled (non-hydrogel based material) NP implants have been investigated by researchers. Both of these NP implants are able to mimic the mechanical behavior of the native nucleus, although they are unable to restore the biomechanical behavior of an IVD.
Discectomy, spinal fusion, and total disc replacement are some of the current surgical treatments to repair degenerated IVD. Discectomy and spinal fusion are complex, costly, and frequently fail to restore the normal biomechanical motion and permanent relieving of the lower back pain of the human spine. There are two types of total disc replacement implants: non-tissue engineered and tissue engineered. Non-tissue engineered total disc replacement implants use a combination of metal and polymers to replacement degenerated disc. There are only two FDA approved implants: SB Charite and ProDisc are currently available in the market. Among the problems of the non-tissue engineered total disc replacement implants are that they often lead to mechanical failure, dislodgement, wear, and associated osteolysis and implant loosening. Tissue engineered total disc replacement implants use the principles of cell biology to determine the nature of the NP, AF and cartilage cells at EP, to create composite scaffolds for each of the components of IVD. The ability of AF cells to remodel and grow in fibrous matrices, as well as the ability of NP cells to assemble hydrogel based extracellular matrix have lead to the fabrication of IVD implants to regenerate AF and NP tissue as a unit. Despite the promise of tissue engineering approaches for design of IVD implants (only one FDA approved implant: Raymedica prosthetic disc nucleus (PDN)), to date no tissue engineered IVD has demonstrated the long term load bearing capability that is equivalent to a native disc. Currently researched tissue-engineered IVD lacks in withstanding the long-term physiological load (cyclic load). Tissue-engineered IVD assembled in the shape of cylindrical disks composed of an outer shell of fiber mesh seeded with annulus fibrosus cells with an inner core of nucleus pulposus cells seeded into gel lack in effectiveness because proper anchorage of the top and bottom sides of IVD is ignored. Bowles et al. in vivo study showed that tissue-engineered IVD, composed of gelatinous NP surrounded by an aligned collagenous AF, can maintain disc space height, produced de novo extracellular matrix, and integrated into the spine, yielding an intact motion segment with dynamic mechanical properties similar to that of native IVD. This study suggested that if a tissue engineered IVD is designed properly, it does not need to be seeded with cells for functioning of the disc. There is no in vivo study reported showing tissue-engineered IVDs withstanding the long-term physiological load. Proper anchorage of NP and AF is required for a successful IVD design, since IVD needs to carry significant deformations even under relatively low-loading conditions and needs mechanical stability for long term physiological loads. Therefore, a tissue and non-tissue engineered IVD construct must include the top and bottom anchorage, which may constitute EP architecture, to NP and AF when engineered IVD is implanted in a human body.
The intervertebral disc (IVD) is one of the body's most vital structures. The nucleus pulposus (NP) in IVD is restricted axially by the superior and inferior cartilaginous endplates (EP) and circumferentially by the annulus fibrosus (AF). It is reported that the mechanical role of the NP is to resist and redistribute compressive normal and shear forces within the spine, whereas the major function of the AF is to withstand tension normal and shear forces. The AF that comprises discrete fibrous sheets with specialized collagen alignment endures multi-directional loads around the circumference. Fibers run in a single direction in native AF tissue, ranging from 20° to 50° with respect to the transverse plane, and adjacent lamellae have opposing fiber orientations, producing an angle-ply structure. Endplates significantly contribute to mechanical characteristic of NP even under relatively low-loading condition. Despite the intensive research over the past decade directed to IVD materials, there is not yet any material that can reproduce adequately the physiological, mechanical and biological behavior of the natural IVD, and at the same time exhibit long-term biomechanical functionality when introduced into the human spine. An IVD implant is needed to restore the spine from instability due to IVD degeneration. A method is needed to provide effective anchorage of an NP implant and improve the mechanical stability of the IVD implant after nucleotomy.